A person just accepting Islam should pronounce
shahaadah before wudu

When a non-believer becomes
Muslim, should he pronounce the two testimonies of faith, al-shahaadatain,
or perform wudu' (ablution) first?

He should say al-shahaadatain
first, then he should purify himself for Salat and he can
perform ghusl (complete bath) because the prophet (peace and blessings of Allah be upon him) ordered
some of his companions to perform it when they accepted Islam.